An atmospheric river headed for California will bring heavy rains to already drenched San Diego County overnight Thursday, creating the potential for flooding, downed trees and large waves to end the first week of the New Year.
The atmospheric river, which is a term used to describe heavier-than-normal rains that draw moisture from the tropics, was already affecting Northern and Central California on Wednesday and was expected to bring potentially damaging rains to San Diego County in the early morning hours Thursday.
Water vapor images via #GOESWest 🛰️ from @NOAA this morning show a powerful storm system over the Pacific bringing strong winds and moderate to heavy rain to the West Coast.
Flooding concerns are widespread across California. Please use caution. pic.twitter.com/CRQcLZe3t2
San Diego Weather Forecast
The system was expected to weaken by the time it hits San Diego County, but will still bring heavy rain across the county through Thursday afternoon. This storm alone could bring up to an inch of rain to parts of San Diego County.
Along with the rain come gusty winds, especially along the coast, where winds will be in the 30 to 35 mph range. A wind advisory is in effect until noon Thursday for the San Diego coast, mountains and valleys.
The mountains will see some snow from this system, but only at the highest elevations. Peaks around 6,000 feet will get dust, but it’s elevations above 7,000 feet that could see anywhere from 6 to 16 inches of fresh powder.
Local
Rain will move away from the region Thursday night with a dry weekend ahead. However, don’t get too used to the sun; another storm is expected early next week, forecasters say.
After the storm is gone, wait for the surf to rise. Waves of about 10 to 16 feet are expected to arrive late Thursday with the biggest swells Friday morning. A high surf warning is in effect from Thursday afternoon through Friday.
That storm could also bring flooding to coastal areas, especially Friday morning, prompting the National Weather Service to issue a coastal flood advisory.
Preparing for Potential Storm Damage
The state predicts the third in a series of atmospheric river storms could be damaging. Trees are already stressed from three years with limited precipitation. Now, suddenly saturated soil and high winds mean they’re more likely to fall, potentially knocking down power lines or creating flooding hazards, said Karla Nemeth, director of the state Department of Water Resources.
Gov. Gavin Newsom on Wednesday declared an emergency in anticipation of potential damage. The declaration allows the state to use the disaster response of the National Guard, ask the Federal Highway Administration for assistance to repair roads and frees up funding for other emergency response needs.
“We anticipate that this may be one of the most difficult and impactful series of storms to hit California in the last five years,” said Nancy Ward, the new director of the California Governor’s Office of Emergency Services.
While the north was expected to receive the brunt of the system, potentially damaging downpours were still expected in San Diego County as of the overnight hours Thursday.
The San Diego County Department of Public Works had crews out Wednesday to clear water mains and roads of debris that could potentially create flooding. For days, workers with plows tended to snowy roads and sandy icy spots. The team also reacted to any fallen trees or rock and landslides from previous storms.
The county encouraged residents to prepare their own homes for the storm by clearing debris that could cause flooding and placing sandbags where necessary (the county and city of San Diego provide free sandbags to residents). Drivers should avoid areas of standing water and flooded roads.
Even after the storm, flooding is possible along the coast as the beaches see waves up to 16 feet. A coastal flood advisory is in effect until Friday.
Crews prepared for possible storm flooding. Del Mar lifeguard Jon Edelbrock said the winds, waves and tides could create the perfect combination for damage to the already fragile bluffs. A team, including members of the Scripps Institution of Oceanography, will monitor the area using a radar system during the storm.
“It’s been a few years since we’ve had something as significant as a surf advisory. I think it’s been five or 6 years, like 2017. The last time we saw kind of a series of swells coming in with this type of size and energy combined with the rain and the coincidence with our spring high tides,” Edelbrock said.
What is an atmospheric river?
Meteorologist Ana Cristina Sánchez explains what an Atmospheric River is and what is the scale for AR measurements.
Three recent storms hitting California this week are considered atmospheric rivers, which is a long narrow band of condensed water vapor that is transported from the tropical regions near the Earth’s equator to the poles, which can produce heavy rain and snow.
They are invisible to the naked eye — the water is moved over the ocean in the form of water vapor, not a “river” as we think of them on land. They tend to move through the atmosphere in streams between 250 and 375 miles wide.
Temperatures at the equator tend to be warmer, and just like in school science lessons, warmer temperatures cause water to evaporate into the atmosphere. Warmer air can also hold more water vapor. When an atmospheric river moves inland and hits mountains, the water vapor rises and cools, producing heavy precipitation.
The strongest atmospheric rivers can move anywhere between seven and 25 times as much water as the flow of the Mississippi River, which is the second longest river in North America and has a watershed that reaches 32 states, according to the national park service.
According to the National Oceanic and Atmospheric Administration, atmospheric rivers “are a major feature of the entire global water supply and flood risks, particularly in the western United States.”
Atmospheric rivers contribute up to 50% of California’s annual precipitation and drive about 84% of flood damage in the western United States. While recent heavy rains have brought some relief to drought-prone California, scientists warn the storms won’t ease up anytime soon. severe deficits in the state’s groundwater and soil moisture.
